Long-latency stretch reflexes of elbow fiexor muscles at different elbow joint angles

Abstract

Stretch stimulus of activated muscle results in a reflex response consisting of short-latency (M1) and long-latency (M2) components. Although the mechanism of the M2 response remains the theme of controversy, at least, it was suggested that the M2 response results from traveling over a long-loop supraspinal pathway. In the present study, reflex electromyographic (EMG) activities of synergistic muscles were recorded at different elbow angles; 20, 50, and 80 deg, for nine subjects. Twenty times stretch stimuli were applied to elbow flexors by a torque motor during submaximal isometric contraction. Reflex EMG activities of m.biceps brachii (BB) and m.brachioradialis (BRD) were divided into four components, i.e., the back ground activities (BGA: -100-0 ms from the onset of the stretch stimulus), short latency reflexes (M1: 20-50 ms), long latency reflexes (M2: 50-100 ms) and Voluntary activities (VOL: 100-150 ms). The iEMGs of M1, M2 and VOL were normalized by BGA at the elbow angle of 50 deg. The results indicated that the relative M2 magnitude of the muscles varied with the elbow angle, and the variation parterns were differed in the two muscles; BB was most activated at 20 deg, inversely, BRD was most activated at 80 deg. From these results, therefore, it was suggested that the central co-ordination patterns in synergistic muscles were depended on joint angles.